The present invention relates to a two-cycle engine and a personal watercraft equipped with the two-cycle engine.
In the prior art, there is a known two-cycle engine as shown in FIGS. 1 and 2. In the figures, FIG. 1 is a partial cross sectional view of the two-cycle and three-cylinder engine. FIG. 2 is a front view of a personal watercraft having the two-cycle and three-cylinder engine mounted on board.
The two-cycle engine of FIG. 2 comprises a crank case 52, a crank shaft 51 to which not-shown pistons are linked and which is rotatably supported by the crank case 52 through bearings J1 (FIG. 1) disposed on a first parting planes L1 of the crank case 52 and three cylinders 53 mounted on the crank case 52. In the crank case 52, the two-cycle engine further includes three intake reed valves 54 juxtaposed in the axial direction of the crank shaft 51, corresponding to the three cylinders, respectively.
As shown in FIG. 1, the two-cycle engine is provided with two balance weights 55 for reducing vibrations caused by reciprocating motion of the pistons. The balance weights 55 are fixed on both ends of a balance rotating shaft 56. The balance rotating shaft 56 is provided, at a substantially intermediate portion thereof, with a driven gear 57 and rotatably supported by the crank case 52 through bearings J2 disposed on a second parting planes L2 of the crank case 52. In operation, the balance rotating shaft 56 is rotated by the crank shaft 51 through a drive gear 58 secured thereon so as to mesh with the driven gear 57. The two-cycle engine further includes an intake-air induction unit 59 containing a carburetor, an exhaust-gas discharging unit 60 containing a muffler, and a starter 61.
Generally, in case of a multi-cylindered engine, a plurality of balance weights have to be arranged apart from each other in the axial direction. From this point of view, the plural balance weights 55 are mounted on the single balance rotating shaft 56 in the above-mentioned conventional two-cycle engine for the personal watercraft.
However, it should be noted that such an arrangement of the weights causes the balance rotating shaft to be lengthened and, the balance rotating shaft should be arranged at a position that is in a circumferential area around the crank shaft and does not interfere with the intake reed valves, the starter or the like.
Consequently, in view of the axial direction of the crank shaft 51, the balance weights 55 are arranged to project radially in circumferentially different positions from the intake reed valves 54 and the starter 61 etc., which arrangement is far from the miniaturization of the engine.
Note, U.S. Pat. No. 3,903,995 discloses a single cylinder engine having two balance rotating shafts, each of which has a single balance weight mounted thereon. However, since the respective balance rotating shafts are so long as to extend from the vicinity of the end of the crank shaft to the radial outside of a crank web, the total length of both balance rotating shafts exceeds to that of the above engine having the single balance rotating shaft.
Again in the two-cycle engine of FIG. 2, the intake reed valves 54 are disposed on the first parting surface 54 running on the axis of the crank shaft 51, while the balance rotating shaft 56 has an axis arranged on the second parting planes L2 of the crank case 52. Thus, since the crank case 52 is divided into three pieces by two pairs of parting planes L1, L2, the structure of the crank case 51 is complicated thereby to increase the manufacturing cost.